Visually Complete aims to indicate what a customer will perceive as the point in time when a site looked complete.
It’s calculated by recording a video of a page loading, which is then broken down into a series of screenshot frames. An image diffing algorithm compares and marks visual completion at the frame with the least amount of difference to the final frame.
Visually complete isn't a faultless metric—sites featuring animating carousels or auto-playing video content can make automated calculation extremely difficult.
If you're looking for a metric that describes when a page is ready for interaction, Time to Interactive may be a better fit.